While the exact mechanisms of ischemia/reperfusion injury remain to be elucidated, inhibition of complement activation (e.g., sCRl or Cl esterase inhibitor), depletion of complement components (e.g., cobra venom factor) and complement component (e.g., C3 and C4) knockout mice have revealed an important role of complement in ischemia/reperfusion (I/R) injury. During the previous two funding cycles we have shown that complement is activated on human endothelial cells following oxidative stress and involves the activation of the lectin complement pathway and mannose binding lectin (MBL). During the last cycle, we identifed C5 as a critical complement component involved in gastrointestinal (G) I/R injury. However, the specific pathways involved, the order of pathway activation, and the specific contribution of C5a versus C5b-9 in the setting of GI/R is not well characterized. Reports in the literature suggest that the classical pathway initiates complement activation and mediates tissue injury in GI/R by way of natural antibody (IgM) deposition. Our preliminary data demonstrate complement activation in GI/R is Clq-independent. Preliminary data raise a new potential role for IgM and MBL interactions, perhaps setting stage for a new paradigm in complement activation. This application will investigate the several gaps in our knowledge about complement and its role in mediating tissue injury and inflammation associated with GI/R. Novel anti-complement reagents and knockout mice have been developed for characterizing the role of each complement pathway in GI/R. Specifically we will identify the importance of each specific complement pathway (e.g., lectin, classical and alternative) in GI/R. Previous reports by our group and others have demonstrated that complement activation influences gene expression and we will identify novel pro- and anti-inflammatory genes that are regulated by complement activation and define the role of C5a vs C5b-9 in GI/R, which seems to differ in the ileum compared to other organs. Novel and potential therapeutically useful reagents will be made and used in vivo to establish and validate the genes expressed in vivo and resolution of inflammation in vivo. Our aims are to: 1) Characterize complement pathways involved in complement activation GI/R; 2) define the role of CSa, C3a and C5b-9 in GI/R, 3) characterize the role of MASPs, IgM and MBL complex regulation in GI/R and oxidatively stressed endothelium, and 4) Define/validate the gene expression profile of the lung and ileum following I/R with an emphasis on complement dependent and independent mechansims.